Clandestine nuclear weapons are an immediate worldwide threat. Rogue nations with nuclear weapons, or terrorist groups acquiring radiological material, could deliver it to a victim nation via commercial shipping. Advanced radiation detectors are necessary to reveal such weapons among shielding and benign clutter. An urgent national priority is the development of radiation detectors that detect and localize shielded radioactive threats.
Although nuclear and radiological threat materials emit gamma rays, heavy shielding greatly attenuates the signal; consequently they are difficult to detect using current detector technology. Gamma rays are detected when they interact with matter via photoelectric absorption in which the gamma is absorbed and a photoelectron is emitted, or Compton scattering which generates a Compton electron and a scattered gamma ray, or electron-positron pair production. In each case, the energetic electron (or positron) can be detected in a charged-particle detector such as a scintillator, which generates light when traversed by the energetic electrons. Gamma rays are blocked or attenuated most effectively by high-density, high-Z material (Z being the atomic number) such as lead.
In addition to detecting the presence of a threat source, it would be highly advantageous to also determine the location of the source. The location information would greatly improve the reliability of the detection, while greatly reducing false alarms. To be most effective, the detector should locate the source in two dimensions, such as horizontal and vertical directions relative to the detector.
Numerous directional radiation detectors have been proposed. Typically they have one-dimensional directionality, meaning that on a single measurement, they can only indicate whether the source is to the left or right of the detector. Then by analyzing multiple measurements taken at different detector orientations, the prior-art detector may be able to specify the source location in one dimension. This is insufficient for large inspection items such as trucks and railcars and shipping containers that extend in both the horizontal and vertical directions. For these and many other inspection challenges, a one-dimensional localization is not enough. Of course a pair of such prior-art detectors could be used to separately scan horizontally and vertically, but this would require two separate systems and would entail some kind of cumbersome coordination between them. Also the two systems would each have its own background rate, further diluting the threat signature and requiring longer scan times. Alternatively, a single prior-art directional detector could scan horizontally first, then roll by 90 degrees, and then scan vertically; but this would take twice as long and would require a complicated mechanical joint.
Prior art also includes numerous imaging-type gamma cameras which can in principle determine the two-dimensional location of a gamma ray source. Gamma cameras typically employ a collimator (pin-hole, multi-aperture, coded-aperture, or other type collimator), which inevitably results in a large, heavy, expensive system yet has low detection efficiency due to losses in the collimator. Prior art further includes pseudo-imaging gamma systems which are based on measuring or imaging the track of a Compton-scattered electron, or they may detect double-scattering of gammas. In either case, the prior-art systems provide only a very approximate source direction at best, yet are even larger and more complex and less efficient than the collimated gamma cameras. Gamma cameras and the others were developed for medical applications, in which a full gamma ray image is needed so that a physician can determine the distribution of a cancer for example. In contrast, most safety and security applications have no need for a gamma ray image; it is quite sufficient to localize a source, raise an alarm, and trigger a secondary inspection.
An advanced gamma ray detector with two-dimensional directionality would be a huge advantage for safety and security applications, because it would greatly speed up the inspection process, would reveal hidden sources with higher sensitivity, and would enable rapid clearing of clean loads automatically. Even more important, the two-dimensional information would greatly enhance the statistical power of the radiation scan, because even a shielded source would be revealed by gamma rays coming from a particular spot. With prior-art non-directional detectors, it is necessary to detect hundreds or thousands of additional gamma rays above background, just to raise a suspicion that a source is nearby somewhere. With a two-dimensional direction detector, on the other hand, an alarm could be raised after detecting just a few gammas coming from the same location in the cargo. In this way, the two-dimensional localization greatly accelerates the scan and greatly amplifies the reliability of the alarm. In addition, the revealed location would provide a valuable starting point for the secondary inspection team. With such a detector, the entire inspection process could be speeded up, resulting in greatly reduced inspection times and reduced entry waits at shipping ports. And more importantly, it would detect a smuggled weapon.
A two-dimensional directional gamma detector would be enabling for many important applications of radiation detection. Walk-through personnel scanners at nuclear facilities would detect contamination as well as pilfering on the spot. Drive-through vehicle and cargo scanners at shipping ports and border crossings would be greatly improved by such a detector. As a portable survey-type instrument, it would enable faster source localization and simpler operation, with reduced radiation exposure to the inspector. As a mobile scanner, of the type used in wide-area searches for hidden nuclear or radiological weapon materials, it would provide improved sensitivity as well as directionality to the search.
What is needed, then, is an integrated gamma ray detector system with two-dimensional directionality. The detector should indicate, on a single measurement, a direction toward the source, thereby assisting inspectors in finding the source. Or, even more preferably, the detector would pinpoint the full two-dimensional source location using just a single data acquisition. Preferably such a detector would be compact, fast, highly efficient, capable of high angular precision, and preferably with low cost.